Soap Bubbles
and the Forces which Mould Them

THE FORCES WHICH MOULD THEM. 29 height gets less, or, what comes to the same thing, because the weight of liquid pulled up at any small part of the curve is always the same. If the plates or the tubes had been made of material not wetted by water, then the effect of the tension of the surface would be to drag the liquid away from the narrow spaces, and the more so as the spaces were narrower. As it is not easy to show this well with paraffined glass plates or tubes and water, I shall use another liquid which does not wet or touch clean glass, namely, quicksilver. As it is not possible to see through quicksilver, it will not do to put a narrow tube into this liquid to show that the level is lower in the tube than in the surrounding vessel, but the same result may be obtained by having a wide and a narrow tube joined together. Then, as you see upon the screen, the quicksilver is lower in the narrow than in the wide tube, whereas in a similar apparatus the reverse is the case with water (Fig. io). I want you now to consider what is happen- ing when twp flat plates partly immersed in water are held close together. We have seen